Last season, Bombardier appeared to have pulled out all of the stops for the latest iteration of its 670cc twin cylinder Mach I. Gas shocks, full instrumentation and the easty-to-operate Bremo hydraulic brake are what separates this potent trail rocket from its little sister, the Formula SS (both now appearing on Ski-Doo's new, lightweight F-2000, all-aluminum chassis). Oh yeah, there are differences in the motor, lots of differences - note, for example, that the cylinders, crankcase and cylinder head will not interchange with previous style 670 twins or the 670 found in the 1995 SS model.
Ski-Doo has built twin-piped engines in the past, the most recent being the Mach IX, the forerunner of the current 670-type engine. The '92 Mach IX was a limited build, twin-piped version of the 617 Mach I motor and was withdrawn from service later that year. Owners of these models were offered a one-time deal to swap cylinders, exhaust and related components changed to '93 Mach I 670 specifications. Of course, in doing so, the twin-piped version disappeared and the single-piped Mach I continued through the '93 and '94 model years and into the '95 season as the Formula SS, Summit 670 and the Grand Touring SE.
The re-appearance of the twin-piped 670 was a welcome development for high-performance trail riders. To accommodate the twin exhaust management capabilities, the engine was fitted with 44mm round slide Mikuni carbs with
(224) AA7 needle jets, a new 7EG06 needle taper and 430/410 main jets. Note
that the stock jetting is richer on the PTO side, a reversal of most Rotax engines of the past, in which the richer jetting was normally found on the MAG side cylinder. An altered crank with a 7.6mm shorter flywheel end (sans flywheel) was stuffed into a new crankcase with a 1.5mm higher deck height. The cylinder head was milled 1.5mm thinner to correct the higher piston top dead center resulting from the deck height change. The new cylinder head has a stamped "1" between the spark plugs to distinguish the new thinner head from the other 670 models. There are now multiple base gasket thicknesses available for this motor, and the ones DYNOTECH checked on crated snowmobiles delivered to its dealership indicates that the factory installs the correct gasket as the motor requires. The thicknesses vary from .013" to .030"+. If the '95 Mach I motor requires a gasket change due to service and repair, it would be advisable to measure the old gasket prior to replacement to ensure the same engine specifications as stock.
A new 501 rotary valve and internal port timing changes complete the twin-piped engine for 1995. With 50 degree temperatures, carbs were fitted with two sizes leaner jetting for our box-stock baseline dyno test. The data would suggest that midrange calibration is conservatively rich, while the main jets are nearly spot on for WOT trail and lake running on pump gas. (The needle clip position for this test and others was the factory calibrated third clip position.) The 1995 Mach I produced a very stout 122.5 CBHP at 8000 RPM.
However, in dyno testing, DYNOTECH discovered that advancing the timing to .085 before top dead center (BTDC) improved the midrange and peak horsepower by 2 hp at mosty RPM, which the dyno-testers thought would be safe for guaranteed 92 octane conventional fuel. The pipes were also a bit hotter on those additional test runs due to shorter cool down times between the dyno runs.
If you make changes along these lines, be advised that you may be in violation of your manufacturer's warranty. Make changes at your own risk.
All dyno testing was conducted by DYNOTECH on its Superflow dyno in Batavia, New York.
1995 SKI-DOO MACH I
Data for 29.92 inches Hg, 60 F dry air
Test: 100 RPM/Sec Acceleration
Fuel Specific Gravity: .740
Vapor Pressure: .30 Barometer: 29.38
| RPM | CBT | CBHP | FUEL | AIR | A/F | BSFC | CAT |
| 5000 | 55.0 | 57.6 | 45.2 | 111.2 | 11.3 | .80 | 48 |
| 5750 | 53.0 | 58.0 | 47.6 | 120.2 | 11.6 | .84 | 49 |
| 6000 | 54.0 | 61.7 | 47.4 | 120.8 | 11.7 | .78 | 49 |
| 6250 | 72.7 | 86.5 | 66.7 | 162.6 | 11.2 | .79 | 49 |
| 6500 | 73.9 | 91.5 | 71.9 | 171.1 | 10.9 | .80 | 49 |
| 6750 | 73.5 | 94.5 | 76.6 | 176.2 | 10.6 | .83 | 48 |
| 7000 | 75.9 | 101.2 | 73.8 | 179.7 | 9.8 | .84 | 49 |
| 7250 | 78.5 | 108.4 | 81.2 | 184.7 | 10.4 | .76 | 49 |
| 7500 | 82.2 | 117.2 | 81.2 | 194.7 | 11.0 | .70 | 47 |
| 7750 | 82.5 | 121.7 | 78.9 | 197.4 | 11.5 | .66 | 48 |
| 8000 | 80.4 | 122.5 | 80.9 | 198.3 | 11.3 | .67 | 48 |
| 8250 | 76.3 | 119.9 | 82.7 | 198.4 | 11.0 | .70 | 48 |
| 8500 | 63.6 | 102.9 | 80.5 | 196.3 | 11.2 | .80 | 47 |
| 8750 | 47.4 | 79.0 | 81.2 | 188.6 | 10.7 | 1.05 | 48 |
RPM: Engine crankshaft speed. CBT: Corrected Brake Torque.
CBHP: Corrected Brake Horsepower. FUEL: Actual fuel flow
pounds per hour.
AIR: Airflow cubic feet per minute A/F: Air/Fuel/ Ratio
BSFC: Brake Specific Fuel Consumption. CAT: Carb Air
Temperature.